1. Field of the Invention
This invention relates to ammonothermal growth of group-III nitrides.
2. Description of the Related Art
Ammonothermal growth of group-III nitrides, for example, GaN, involves placing, within a reactor vessel, group-III containing source materials, group-III nitride seed crystals, and a nitrogen-containing solvent, such as ammonia, sealing the vessel and heating the vessel to conditions such that the vessel is at elevated temperatures (between 23° C. and 1000° C.) and high pressures (between 1 atm and, for example, 30,000 atm). Under these temperatures and pressures, the nitrogen-containing solvent may become a supercritical fluid which normally exhibits enhanced solubility of the group-III containing source materials into solution. The solubility of the group-III containing materials into the nitrogen-containing solvent is dependent on the temperature, pressure and density of the solvent, among other things. By creating two different zones within the vessel, it is possible to establish a solubility gradient where, in one zone, the solubility will be higher than in a second zone. The group-III containing source materials are then preferentially placed in the higher solubility zone and the seed crystals in the lower solubility zone. By establishing fluid motion of the solvent with the dissolved source materials between these two zones, for example, by making use of natural convection, it is possible to transport the group-III containing source materials from the higher solubility zone to the lower solubility zone where the group-III containing source materials are deposited onto the seed crystals.
The current state of the art uses a device or vessel that contains the supercritical solvent and this vessel is heated to raise the entire vessel contents to elevated temperatures and pressures. The heating of the vessel is commonly performed by heating the outer walls of the vessel and, by virtue of heat transfer, heating the inner walls of the vessel, which, in turn, heats the solvent, group-III containing source materials, group-III nitride seed crystals and other material present within the vessel.
For the ammonothermal growth of group-III nitride crystals, it is important to establish solubility gradients. One method of producing these gradients is to establish temperature gradients since the solubility is a function of temperature, among other variables. Therefore, it is crucial for the growth characteristics of the group-III nitride crystals to establish well defined and controllable temperature gradients across well defined and placed spatial zones within the reactor.
In addition, many current reactor designs for the ammonothermal growth of group-III nitride crystals involve placing the group-III containing source materials and group-III nitride seed crystals within a vessel, where the vessel has a small ratio of inner diameter to length of vessel. By virtue of this design, the vessel has a long cylindrical shape wherein the group-III containing source materials or group-III nitride seed material are preferentially placed in different zones of the vessel, and the different zones are often separated by the use of baffle plates, which function as restriction devices for the fluid flow of the solvent, so it is possible to establish temperature gradients between the two zones.
Thus, what is needed in the art are new reactor vessel designs for use in ammonothermal growth of group-III nitride crystals. Specifically, what is needed in the art are improved techniques for heating reactor vessels used in ammonothermal growth of group-III nitride crystals. In addition, what is needed in the art are improved baffle designs for reactor vessels used in ammonothermal growth of group-III nitride crystals. The present invention satisfies these needs.